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TDs were seen scrambling for their legal dictionaries last week when the legal concept of ‘obiter dictum’ became central to the debate on the Protection of Life During Pregnancy Bill.

Retired Supreme Court Judge Hugh O’Flaherty - one of the judges who originally heard the X case - recently commented that aspects of the X case decision may be obiter dictum - that is, not binding on later courts. This was picked up by TDs opposed to the legislation.

Obiter dictum is a Latin phrase meaning ‘things said by the way’. Future courts reviewing a judicial decision may be persuaded by comments made obiter dictum, but they are not bound to follow that rationale.

By contrast, the parts of the decision that are ‘ratio decidendi’ are binding on later lower courts. Ratio decedendi is a Latin phrase meaning ‘the rationale for the decision’. The ratio decidendi is “the point in a case which determines the judgment” or “the principle which the case establishes” - it is the essentially meat of the judgement: the reason why the decision was made.

When seeking to argue any given case, a lawyer will search for relevant precedent cases and will read through the judgements carefully, seeking to distil the ratio decidendi, which he may use to bind the court to decide in a certain way.

Judges in delivering judgements do not conveniently label parts of the decision ratio decidendi or obiter dictum. It is for lawyers to later discern and argue which is which. The matter is complicated when several judges deliver slightly different reasoning.

To illustrate the difference between obiter dictum and ratio decidendi, imagine the following scenario: John erects a flagpole in the front garden. He gets planning permission for it and has an engineer construct it to the highest possible standards. It is struck by a meteorite and falls on his neighbour’s house, damaging it. In the subsequent legal case, the judge decides that a flagpole being struck by a meteorite is incredibly rare, and so is not reasonably foreseeable. Therefore, John is not liable.

However, the judge mentions in passing that if it had been struck by lightning, instead of a meteorite, and it had not been properly built to withstand lightning, he would have regarded the damage as reasonably foreseeable because flagpoles are well-known to be struck by lightning on occasion.

Many years later, in a different town, Mary puts up a flagpole, without planning permission. It is shoddily built and has no lightning conductor to help it withstand a lightning strike. It is duly struck by lightning and falls, damaging her neighbour’s house. After searching through many dusty legal books, Mary’s neighbour’s lawyer finds the comments made obiter dictum in the previous case taken against John, years before. He zones in on the judge’s passing remark that if the flagpole had been struck by lightning, liability could have arisen, as lightning is a reasonably foreseeable risk to flagpoles. The court is persuaded by these earlier judicial comments made obiter dictum, and decides that Mary should be liable as lightning strikes are a foreseeable risk to flagpoles, and it is negligent not to construct it to withstand a strike.

Now, in the case against Mary, the court has decided ratio decidendi that lightning strikes on flagpoles are reasonably foreseeable, and if the pole is negligently constructed then the owner is be liable for any resulting damage. What was once merely a passing, hypothetical remark by a judge years earlier - obiter dictum - has become a ratio decidendi for a new decision and so is becomes law, and is binding on lower courts. In this way, case law has evolved organically over decades and even centuries.